Method of producing magnesium and magnesium alloys



Aug. 30, 1955 E. F. EMLEY ET AL 2,716,603

METHOD OF PRODUCING MAGNESIUM AND MAGNESIUM ALLOYS Filed March 4, 1952 firfform 45 nite METHUD 0F PBJDDUCING MAGNESIUM AND MAGNESIUM ALLOYS Application March 4, 1952, Serial No. 274,748

Claims priority, application Great Britain March 8, 1951 6 Claims. (Cl. 75-67) This invention relates to the production of cast and wrought articles of magnesium and magnesium base alloys.

It has been a constant desire in the industry for many years to improve the corrosion resistance of such articles. It is well known that magnesium base alloys contain very small particles of impurities which act as centers of initiation of corrosion. These particles are generally considered to contain iron. For example, in a well known type of alloy containing aluminium zinc and manganese the particles may contain aluminium, manganese and iron, and in the case of another known type of alloy containing zirconium the particles may contain aluminium, manganese, silicon and iron, in addition to zirconium.

Attempts have been made to remove iron from molten magnesium alloys by the addition of precipitating agents such as manganese, beryllium and zirconium but these methods have only proved successful to a limited and variable degree. Moreover when the removal of the iron has been effective, the expected improvement in corrosion resistance is not always achieved. Other methods have also been attempted for improving the corrosion resistance of magnesium alloys including the use of magnesium which has been sublimed and the use of special crucibles to avoid iron pick-up during melting. These methods also are not wholly successful and in addition are for most purposes impracticable owing to their inconvenience and expense.

According to the present invention the magnesium or magnesium base alloy is introduced into a container which is rotated at a speed high enough to produce a comparatively sharp separation of the undesirable particles; the rotation is continued until the alloy solidifies whereupon it is removed from the container, and the outer skin containing the particles is removed.

The article so produced may take the form of a casting of substantially finished shape or it may be of a shape suitable for rernelting and casting by sand casting, gravity die casting, or pressure die casting. Alternatively the article may be used for the production of wrought goods by plastic deformation e. g. rolling extruding or forging.

The container constitutes a mould which may be in the form of a sand mould, or a permanent metal mould, or any other suitable type of mould.

The container or mould may be formed of special shape at the outermost parts so that the solidified article may have projections or the like containing the particles to facilitate machining off the unwanted material.

We have found that, for example, with alloys containing magnesium and manganese, and with alloys containing zirconium, a sharp separation of the undesirable particles can be obtained at practicable speeds. For example, with a container having an 8 inch diameter and about 4 inch axial length a speed or 950 revolutions per minute resulted in practically the whole of the undesirable particles being contained within a depth of of the outer part of the solidified block. It will be appreciated that States Patent with a container of a larger diameter, a less number of revolutions per minute will be suitable.

Substantially the whole of the particles will in general be contained within a layer of skin having a depth of at least one eighth of an inch and probably not exceeding one half of an inch.

The alloy may be poured into the container in molten form or it may be melted in the container. The period during which the container is rotated will generally not be less than ten minutes and may be up to twenty minutes or more, depending on the size of the block and initial temperature of the alloy.

The alloy may be heated to a comparatively high temperature initially in order to increase the separation period before the alloy solidifies. For example, a temperature of 850 C. will be suitable for most alloys. Alternatively, the container itself may be heated while the metal is rotating. The container need not be concentric with the axis of rotation. For example the container may be cylindrical with the longer axis perpendicular to the axis of rotation and the axis of rotation passing through the axis of the cylinder but located outside the cylinder.

If desired the container may be suitably constructed or treated to minimise iron pick-up. For example, it may be made of steel containing aluminium together with chromium and/or manganese as described in the specification of British Patent No. 649,594. Alternatively the container may be made of graphite in which case it may be provided with an external metal housing. As a further alternative, the container may be made of cast iron treated internally with a suitable wash or metallic coating, e. g., it may be aluminisecl by spraying with aluminium.

Provision may be made for facilitating removal of the block from the container after solidifying. For example, the container may be made of tapered form and/ or may be treated internally with a wash which militates against adhesion of the block to the container. Alternatively, the container may be made of sheet iron, e. g., of the order 1 to 3 millimeters thickness so that it can be removed by cutting it off after casting.

The invention is of particular importance in relation to magnesium base alloys containing zirconium because the corrosion resistance of these alloys cannot be improved merely by exclusion of certain impurities such as iron and nickel which are well known to affect adversely the corrosion resistance of magnesium and its alloys containing aluminium zinc and manganese. Thus it is possible to improve the corrosion resistance of Mg-Al-Zn-Mnalloys by using metals of high purity in order to produce an alloy low in iron and nickel. This method however has in practice not produced any improvement in corrosion resistance with magnesium-zirconium alloys. The magnesium-zirconium alloys may contain other alloying r constituents consisting of elements which in molten condition do not combine with zirconium to form compounds insoluble in the magnesium as described in the specification of applicant companys British Patent 511,137, i. e., the alloy may include any one or more of the elements zinc, cadmium, cerium, silver, thallium, thorium, copper, bismuth, beryllium, lead, and calcium but excludes the elements aluminum, silver, tin, manganese, cobalt, nickel, and antimony.

If desired, when applying the present invention to magnesium alloys containing zirconium some proportion of a known alloying composition capable of introducing zirconium into magnesium may be present in the container so as to offset losses of zirconium from solution during centrifuging. Such alloying compositions consist of halides of zirconium together with other halides, and after reaction with the magnesium, these separate to the outer parts of the casting together with the aforementioned particles of impurities and are removed therewith. If desired the bulk of these halides may be removed prior to machining, e. g., by washing or immersing in an aqueous solution of chromic acid.

Wrought stock made in accordance with the present invention has shown a surprising increase in corrosion resistance as indicated in the following experiments.

EXPERIMENT I 1.0 lbs. of commercially pure magnesium were melted in a mild steel crucible and saturated with zirconium at 800 C. by the method disclosed in the specification of British Patent No. 652,227. Part of the alloy was transferred to a cylindrical container of steel sheet 8" in diameter and 3 high which was placed in a preheated refractory-lined casing covered with a lid and rotated about a vertical axis so as to throw the particles of impurities to the walls of the container. The rotation was maintained at approximately 950 R. P. M. for 15 minutes after which time the metal was completely solid. Having stripped the solid block of metal from the container, it was found that the particles were concentrated in the outer layers of metal to a depth of approximately 7 The particles were removed by machining and corrosion test specimens cut from the metal. A second portion of the same melt was transferred to a clean mild steel crucible, and held quiescent for 15 minutes, and chill cast into test bars to provide control test specimens.

The corrosion test results obtained in four melts as described are shown in Table I.

Table I.C0mparative corrosion rates of centrifuged and normally cast Mg-Zr alloy n a, H

Corrosion Rate (Weight; loss in 28 days total immersion in 3% NaCl. Mean of 3 speci- Melt NO mens-mg./cm. /day) Normally Cast Centrifuged Control Metal The beneficial effect of centrifuging on iron removal is illustrated in the case of Elektron (registered trademark) A8 alloy containing 8% aluminium, 0.5% zinc, and 0.25% manganese in Table II.

Table II.-C0mparative iron contents of Elektron A8 alloy when centrifuged and normally cast Iron Content (Percent) Melt No.

Centrifuged Normally Metal Cast Control The invention is also of particular importance in relation to wrought stock for the fabrication of etching plates for printing, since the presence of particles of impurities embedded in the surface of the etching plates has a deleterious effect upon the quality of the work produced therefrom. In applying the present invention to the production of etching plates the metal from which the plates are to be fabricated may itself have been centrifuged as described, or alternatively a hardener alloy of magnesium with manganese or zirconium may first be prepared substantially free from particles by centrifuging as described and this hardener alloy may then be incorporated in a melt of magnesium or magnesium base alloys. In this manner alloys of magnesium containing manganese or zirconium may be made which are substantially free from the undesirable insoluble particles rich in manganese or zirconium hitherto present in magnesium base alloys containing these elements.

The invention will now be further described by way of example with reference to the accompanying diagrammatic drawings wherein:

Figure 1 is a view partly in elevation and partly in vertical central section of a centrifugal casting device for use in carrying out the invention;

Figure 2 is a view partly in elevation and partly in section of a cast hollow block or billet of magnesium base alloy made in the device shown in Figure 1;

Figure 3 is a plan view of the block; and

Figure 4 is a plan view of an alternative shape of mould.

In Figure 1 a vertical shaft 10 carries a steel casing 11 having a refractory lining 12. Within the lining is a cylindrical steel mould 14 having a neck 19 of reduced diameter. The casing 11 has a flange welded to its upper edge and a pair of uprights 16, 17, which carry a crossbar 18. A clamp screw 20 is threaded into a tapped hole in the crossbar and presses a cap 21 down on to the flange 15.

The molten magnesium or magnesium alloy to be treatedv is poured into the mould 14 to fill it up to the base of the neck 19 and the cap 21 is then applied. The shaft 10 is then rotated for a period long enough to enable the metal to solidify. The shaft 10 is then stopped and the cap and mould removed and the mould is cut up to remove the cast block 24 (Figures 2 and 3). This block has the zirconium, manganese or other particles 25 in the outer cylindrical layer 26. This layer is then machined off on a lathe leaving the block ready for further shaping by casting, machining, or plastic deformation.

In Figure 4 the plan outline is a mould which is rotated about an axis 3233 and has tapered ends 28, 29 forming projections 30, 31, on the cast block which will contain the particles. These projections are cut off on the planes 34-35, 36-37 leaving the block 24 ready for further working.

We claim:

1. A process for improving the corrosion resistance of magnesium base alloys containing zirconium wherein the magnesium base alloy is introduced into a container which is rotated at a speed high enough to produce a comparatively sharp separation of the undesirable suspended particles in the alloy, the rotation is continued until the alloy solidifies whereupon it is removed from the container, and the outer skin containing the particles is removed.

2. A process as claimed in claim 1 wherein the alloy contains at least one element selected from the group consisting of zinc, cadmium, cerium, silver, thallium, thorium, copper, bismuth, beryllium, lead, and calcium.

3. process of making castings which consists in remelting the machined alloy formed in accordance with claim. 1 and recasting it.

4. A process. as claimed in claim 1 wherein the alloy in the container contains an alloying composition capable of introducing zirconium into magnesium.

5. A process as claimed in claim 1 wherein the machined alloy is subsequently shaped by plastic deformation.

6. A method of producing articles of magnesium base alloy wherein a magnesium base alloy produced in accordance with claim 1 is melted with a further quantity of magnesium and reca References Cited in the file of this patent UNITED STATES PATENTS 1,559,203 Thomson Oct. 27, 1925 2,107,513 Swoger Feb. 8, 1938 2,404,364 Cooper et al. July 23, 1946 

1. A PROCESS FOR IMPROVING THE CORROSION RESISTANCE OF MAGNESIUM BASE ALLOYS CONTAINING ZIRCONIUM WHEREIN THE MAGNESIUM BASE ALLOY IS INTRODUCED INTO A CONTAINER WHICH IS ROTATED AT A SPEED HIGH ENOUGH TO PRODUCE A COMPARATIVELY SHARP SEPARATION OF THE UNDESIRABLE SUSPENDED PARTICLES IN THE ALLOY, THE ROTATION IS CONTINUED UNTIL THE ALLOY SOLIDIFIES WHEREUPON IT IS REMOVED FROM THE CONTAINER, AND THE OUTER SKIN CONTAINING THE PARTICLES IS REMOVED. 